Several apparatus have been proposed to prevent the filling of automobiles with a wrong fuel by the driver or filling operator at refueling facilities such as filling stations. Examined Japanese Patent Publication (kokoku) Hei 4-64958 discloses an apparatus that uses a gas sensor such as one of a contact burning type; according to the teaching, the fuel vapor in the fuel tank of an automobile to be refueled is aspirated and its concentration is measured to identify the type of the fuel used by the automobile, thereby preventing from filling with a wrong fuel. The device identifies the fuel of interest as gasoline if the measured concentration of the fuel vapor exceeds a preset value or as gas oil (diesel gas oil) if the measured concentration is less than the preset value.
A problem with this apparatus is that since the inlet to the fuel tank remains open to the ambient air atmosphere during refilling, the concentration of the aspirated fuel vapor may potentially be influenced to a great extent by external factors such as weather conditions. If this occurs with a gasoline-fueled car, the gasoline vapor may be diluted with air, causing the apparatus under consideration to read a lower value than it should otherwise and conclude that the automobile to be refueled is a gas oil-fueled car. This possibility cannot be avoided as long as one takes the approach of measuring the absolute values of fuel vapor concentration.
Unexamined Published Japanese Patent Application (kokai) Hei 2-85200 discloses a method for distinguishing a particular oil by the difference in the exhaust sound that develops when the aspirated fuel vapor is discharged through a mechanism having a structure like that of a wind instrument. This method requires that the fuel vapor to be identified be present in a concentration higher than a certain level since the difference in exhaust sound originates from the difference between the densities of two fuel vapors. Hence, the reliability of the method is not assured if the fuel vapor of interest has a very low concentration.